1. Field of the Background
The invention relates to optical waveguides and to waveguide devices such as optical switches such as used in telecommunications. The invention is in more specific terms concerned with optical waveguides comprising poly-4-vinylpyridine.
2. Discussion of the Background
As communication and data transfer systems increasingly employ optical fibres, there is a growing requirement for optical and electro-optic devices for switching purposes.
Many materials have been investigated for use in such devices. For example, it has been found that some liquid crystal materials can be used in optical switches but these have tended to suffer from the disadvantage that switching is too slow.
Inorganic material such as lithlum niobate have been used but are difficult to prepare in a satisfactory form.
Molecular weight organic materials in optical and electro-optic devices has been investigated over recent years. However, there are significant problems which still remain, both in predicting the values of non-linearities for organic crystals and in achieving the necessary crystal structures. Despite the fact that considerable success has been achieved in producing organic crystals with good non-linear optical properties and resistance to radiation damage, there are serious problems in the fabrication of the thin single crystal films required.
In contrast, polymeric materials can easily be fabricated into thin films and this has resulted in a growing interest in their application to integrated optics. Methods have been devised for the photo-fabrication of low loss channel guides, particularly in polymethylmethacrylate and also the production of multilayer optical circuits (see, for example, L Kato and M Komatsu, Elect. and Commun. in Japan, No. 66-C, 106 [1983]). Emphasis has recently been given to the non-linear optical properties of polymeric optical guides incorporating loadings of organic molecules having large optical non-linearities (see, for example, G Elliott, MRC, ITM 84/2). The loaded polymers are usually prepared from a solution of the polymer and the material to be loaded into the polymer. Alternatively, it is possible to employ as the material to be loaded a monomer which can subsequently be polymerized into the overall polymeric structure. Of course, the material to be loaded can also be diffused into a preformed polymer, although this method suffers from the disadvantage that small quanities only of the active loading material can be incorporated into the polymer in this way. Evaporation of a mixed solution generally results in the production of a polymeric glass having a uniform distribution of active material.
In some system, however, the materials used undergo phase segregation even at low concentrations. This limits the usefulness of the polymeric materials concerned due to the imposed limitations on the capacity of the polymeric material to suffer variation in refractive index in an electric field. Polymethylmethacrylate is probably the most widely studied polymeric matrix material. G Khanarian et al, Proc SPIE 682, 153, 1987 discloses that the maximum loading of 2-methyl-4-nitroaniline in polymethylmethacrylate is 10% by weight. Active waveguides using polycarbonate with a 65% by weight loading of R-(+)-5-nitro-2-[N-(1-phenylethyl)-amino]pyridine (MBA-MP) have been reported although this level of loading for polycarbonate is by no means universal.
The incorporation of a dye in a thin film polymeric waveguide is known to provide an increase in refractive index. The use of dyes in this way has, however, been limited by the difficulty in incorporating substantial amounts of dyes in polymer films. It has been found that, for example, the larger thiapyrilium dye molecules segregate from polycarbonate to some extent at loadings of 1.5% by weight.